not sure what i am doing here but here's what I did:

1: upgraded to kernel 2.6.6 - compiled make install ran NVidia installer - all ok.
2: now using new kernel - can't compile new programs - realised I haven't upgraded the headers.
3: removed 2.4.26 headers package then copied:

asm-generic
asm-i386
linux
asm (link to asm-i386)

FROM the untouched original source in /usr/src/linux-2.6.6
TO /usr/include

4: recompiled 2.6.6 kernel - all ok
5: tried to recompile MPlayer, ./configure --enable-gui worked ok but when I tried to "make" it got the this error (last few lines):

make -C loader
make[1]: Entering directory `/home/tuttle/sources/MPlayer-1.0pre4/loader'
cc -I. -I.. -I../libvo/ -I../../libvo -O4 -march=athlon-xp -mcpu=athlon-xp -pipe -ffast-math -fomit-frame-pointer -D_REENTRANT -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -I/usr/include/gtk-1.2 -I/usr/include/glib-1.2 -I/usr/lib/glib/include -I/usr/X11R6/include -U_FILE_OFFSET_BITS -I/usr/include/gtk-1.2 -I/usr/include/glib-1.2 -I/usr/lib/glib/include -I/usr/X11R6/include -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -DMPLAYER -D__WINE__ -c ldt_keeper.c
ldt_keeper.c:28:27: linux/version.h: No such file or directory
ldt_keeper.c:29:41: missing binary operator before '('
ldt_keeper.c: In function `Setup_LDT_Keeper':
ldt_keeper.c:175: storage size of `array' isn't known
ldt_keeper.c:208: sizeof applied to an incomplete type
make[1]: *** [ldt_keeper.o] Error 1
make[1]: Leaving directory `/home/tuttle/sources/MPlayer-1.0pre4/loader'
make: *** [loader/libloader.a] Error 2

I also reinstalled most of the packages from the slackware/d/ directory on the disk to see if that helped, at first I messed things up and the system couldn't find the "as" comand so I installed the package (previously removed during a dropline update) which provides "as" command - "binutils" but now I seem to be back at square one!
I know I've done something really stoopid so please be gentle!!

edit: I didn't link /usr/src/linux-2.6.6 to /usr/src/linux so I did this with no effect
also, I got supertux to compile ok, does that mean mplayer is playing up?!
sorry, forgot to mention that I haven't tried to compile this version of mplayer (1.0pre4) before.

Do the sources come from www.kernel.org ? If no, download a 2.6.6 source kernel, untar it in /usr/src then rm linux, ln -s linux-2.6.6 linux

Thanks for reply keefaz, that's how I did it, and tried the link thing (see my edit), to no avail. I realise that this may not even be connected to the kernel but it was a new exercise for me so I'm really asking whether or not I am doing things right.

I've found another error in the make log:

make[2]: Entering directory `/home/tuttle/sources/MPlayer-1.0pre4/libvo'
cc -MM -I../libvo/ -I../../libvo -O4 -march=athlon-xp -mcpu=athlon-xp -pipe -ffast-math -fomit-frame-pointer -D_REENTRANT -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -I. -I.. -I../osdep -I/usr/include/freetype2 -I/usr/include/SDL -D_REENTRANT -I/usr/X11/include -DMPG12PLAY geometry.c aspect.c aclib.c osd.c font_load.c gtf.c spuenc.c video_out.c vo_null.c vo_pgm.c vo_md5.c vo_mpegpes.c vo_yuv4mpeg.c vo_tga.c x11_common.c vo_x11.c vo_xover.c vo_xv.c vo_dga.c vo_gl.c vo_gl2.c vo_mga.c vo_xmga.c vo_aa.c vo_svga.c vo_fbdev.c vo_fbdev2.c vo_png.c vo_jpeg.c vo_gif89a.c vo_vesa.c vesa_lvo.c vo_sdl.c vo_cvidix.c vo_xvidix.c sub.c font_load_ft.c vosub_vidix.c 1>.depend
In file included from vo_mga.c:17:
../osdep/kerneltwosix.h:1:27: warning: linux/version.h: No such file or directory
../osdep/kerneltwosix.h:2:41: missing binary operator before '('
In file included from vo_fbdev.c:21:
../osdep/kerneltwosix.h:1:27: warning: linux/version.h: No such file or directory
../osdep/kerneltwosix.h:2:41: missing binary operator before '('
In file included from vo_fbdev2.c:16:
../osdep/kerneltwosix.h:1:27: warning: linux/version.h: No such file or directory
../osdep/kerneltwosix.h:2:41: missing binary operator before '('
In file included from vo_jpeg.c:16:
../config.h:64:1: warning: "HAVE_STDDEF_H" redefined
In file included from /usr/include/jpeglib.h:24,
from vo_jpeg.c:14:
/usr/include/jconfig.h:11:1: warning: this is the location of the previous definition
make[2]: *** [depend] Error 1
make[2]: Leaving directory `/home/tuttle/sources/MPlayer-1.0pre4/libvo'

something missing perhaps? libvo hmm...

edit: just tried linking /usr/src/linux to /home/tuttle/sources/linux-2.6.6 & still no workee!

I guess the first error line :
../osdep/kerneltwosix.h:1:27: warning: linux/version.h: No such file or directory

shows that the program is not in good directory for linux headers, eg in my system :

$ locate /linux/version.h
/usr/src/linux-2.4.22/include/linux/version.h
/usr/src/linux-2.6.5/include/linux/version.h
/usr/include/linux/version.h
so ./configure your mplayer --[with proper include option] /usr/src/linux/include
and see if it works

aaaaaaaaargh (in surprise and alarm) I copied include/linux from /usr/src/linux2.6.6 instead of /home/tuttle/sources/linux-2.6.6 (where I compiled it)

Case solved. Cheers for the help :~]